The present invention relates to a device and method for detachably connecting an impeller member to a pinion shaft member in a high speed fluid compressor, and more particularly the invention relates to a connection device and method where one of the members includes at least one tab that is inserted into a corresponding at least one slot provided on the other member.
A high speed fluid compressor such as a centrifugal compressor includes a rotor assembly that is comprised of an impeller that is coupled to a pinion shaft which includes a pinion gear that meshes with a drive gear to drive the impeller at high rotational velocities of up to 76,000 rpm, for example. The suitable attachment between the impeller and pinion must be able transmit torque from the pinion gear to the impeller, maintain zero relative motion of the impeller relative to the pinion, permit easy assembly and disassembly of the rotor assembly, and consistently relocate the pinion and impeller at their original relative positions when the components are reassembled. Accurate maintenance of the relative positions of the impeller and rotor is critical to ensure that the rotor assembly retains its dynamic balance.
The impeller and pinion shaft are conventionally coupled by a polygon attachment method. The principal advantages of the polygon attachment method are its ease of assembly/disassembly and self centering characteristic. The polygon must consistently lock up the impeller and pinion shaft at the same position to maintain the needed level of rotor balance. Any relative movement between the pinion shaft and impeller leads to unacceptable levels of vibration during compressor operation. To ensure the requisite consistency is obtained, the mating parts must be machined to very exacting tolerances.
FIG. 1 illustrates a prior art rotor assembly generally comprised of pinion shaft 12 coupled to an impeller 14 by a polygon attachment method. The pinion shaft 12 includes pinion gear 16 which is engageable with a power transmission assembly (not shown) which drives the pinion about a pinion axis 18 at a predetermined rotational velocity during operation of the centrifugal compressor. The pinion shaft 12 includes a drive end 20 which has formed therein a polygonally dimensioned bore 22. The polygonally dimensioned bore 22 has an interior bore surface which defines a generally triangular cross section composed of circular arcs.
The impeller 14 incorporates a backward-leaning type blade geometry 24, and the impeller includes a polygonally dimensioned stem portion 26 which is defined by an exterior stem surface 28. The stem portion 26 includes a first end 26a and a second end 26b. The polygonally dimensioned stem portion 26 is suitably matingly dimensioned to be received by the polygonally dimensioned bore 22. The stem portion 26 is typically dimensioned to have a cross section which deviates from a circular pattern and which has a shape that is convex on all sides and essentially elliptical, triangular or quadratic as illustrated in FIG. 2. After coupling the pinion shaft and impeller, the pinion shaft is rotated and the lobes along the stem 26 are locked against adjacent portions of bore 22.
The polygon attachment method has a number of shortcomings. The polygon attachment method is useful because it is repeatable and maintains permanent location by its shape. However, if the mating parts are not parallel and the shapes of the lobes are not accurately calculated and precisely machined, as the rotor assembly comes up to speed stresses in the components may alter the shapes of the lobes and as a result loosen the connection between the pinion shaft and impeller. Also, the compressor could experience surge or vibration that occurs during operation and as a result the surge or vibration could displace the impeller to a new location and out of balance. The polygon is expensive and difficult to manufacture. The mating polygon surfaces are difficult to measure for quality and precision. The continuous rubbing and surface contact on highly stressed polygonally shaped parts causes galling and fretting of the parts and the galling and fretting could cause the impeller and pinion shaft to be fused together.
The foregoing illustrates limitations known to exist in present devices and methods for assembling impellers and pinion shafts. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the present invention, this is accomplished by providing a rotor assembly that includes an impeller including an impeller stem, the stem including a first coupling end having a first face and first coupling means along the first face; and a pinion shaft having a second coupling end with a second face and second coupling means along the second face, the first and second coupling means adapted to be mated when the impeller and pinion shaft are assembled to prevent relative displacement of the impeller and pinion shaft.
The first coupling means is comprised of at least one arcuate tab, and the second coupling means is comprised of at least one arcuate slot adapted to receive the at least one arcuate tab when the impeller stem and pinion shaft are mated. Each tab includes an inner arcuate surface, and substantially planar terminating surfaces joining the inner and outer arcuate surfaces; the arcuate tabs having different arclengths and widths. If one tab is included, the tab is simply inserted into the mating slot, and if more than one tab is provided, the tabs are different with different arclengths so that they can only be inserted into their mating slot and in this way the required relative orientation between the stem and pinion shaft is maintained.
In addition to the tab/slot coupling structure the pinion shaft includes a hub that extends outwardly from the second face and is adapted to be mated with a bore formed in the impeller stem. The wall of the bore and hub are tapered so that an interference fit is created when the hub is inserted in the bore.
In summary, the present invention is comprised of an attachment device and method comprised of a set of tabs/slots and tapered cylindrical hub. The tab/slot feature is used to transmit power between the mated parts and the tab/slot feature limits assembly of the component parts to a single orientation ensuring that the pinion shaft and impeller will be assembled at the same relative position when the parts are disconnected and then reassembled. The tapered cylindrical hub achieves an interference fit between the mating parts, and thus ensures that the two mating parts do not move relatively in the radial dimension. This ensures retention of dynamic balance of the assembly. Also, the interference fit that is achieved, provides additional power transmission capability. This design provides means to achieve the needed joint stiffness, balance retention, and power transmission capabilities while it can more easily be manufactured than the conventional polygon and other attachment methods.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.